Rechargeable batteries, such as a lithium ion battery, are widely used as driving batteries for a variety of devices, such as a cellular phone, a notebook computer, and the like, due to particular battery characteristics, such as having a large capacity, a high energy density, and ability to efficiently charge and discharge.
Lithium ion batteries are generally charged by a combined constant current and constant voltage technique. In particular, the battery may be initially charged with a constant current until the battery voltage reaches a predetermined voltage value. Once this voltage is reached, the charger maintains a constant charge voltage as the current decreases. Charging is complete when the current reaches a terminal current value.
Many battery systems are equipped with circuitry to compute the remaining capacity of the battery (typically measured in Amp-hr). The remaining capacity may be expressed as a percentage of the full charge capacity (maximum capacity) of the battery. This percentage value is referred to as the relative state of charge (RSOC) of the battery. Electronic devices typically display this percentage value to inform a user of the amount of stored energy available for use.
The full charge capacity of the battery, however, decreases with age and cannot be easily measured. Many battery systems have various circuits and components to estimate the full charge capacity of the battery but these estimates contain a margin of error. This margin of error is then propagated to the RSOC value. Conventional battery systems may also employ look-up tables to adjust the RSOC value in an attempt to correct the error. Look-up tables, however, occupy a large storage area and require more power to retrieve data from the look-up table.
Battery systems generally compute the RSOC value of the battery using different methods depending on the charge mode (constant current charge mode or constant voltage charge) of the battery. It is during the switch from one mode to another that the RSOC error is most noticeable.